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Astron. Astrophys. 336, 840-854 (1998)
Stellar disks of optically flocculent and grand design spirals *
Decoupling of stellar and gaseous disks
P.J. Grosbol 1 and
P.A. Patsis 2, 3
1 European Southern Observatory,
Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany
2 Max-Planck Institut für Astronomie, Königstuhl
17, D-69117 Heidelberg, Germany
3 Astronomisches Rechen Institut, Mönchhofstraße
12-14, D-69120 Heidelberg, Germany
Received 24 March 1998 / Accepted 8 June 1998
Abstract
Accurate surface photometry of five spiral galaxies (NGC 3223,
NGC 5085, NGC 5247, NGC 5861 and NGC 7083) in
visual and near-infrared bands is presented. All galaxies show grand
design spiral structure in K![[FORMULA]](img1.gif)
although several
have a flocculent appearance on blue images suggesting that stellar
and gaseous disks are decoupled. The decomposition of the
K maps in axisymmetric components suggests that
both a spherical bulge and a flat exponential disk are required to
explain the light distribution in the bulge regions. The central disk
has a short scale length and sky projection parameters similar to the
main disk. The scale length of the main exponential disk measured in
the interarm region is consistently smaller than the value determined
from the arms for three of the galaxies. Weak oval distortions were
found in three galaxies while only one was classified as such. The
galaxies with oval distortions have less concentrated and fainter
bulges than those in the two galaxies without such ovals.
Model rotation curves were constructed for each galaxy based on the
axisymmetric decomposition of their K surface
brightness maps and observed velocity data. The mass-to-light ratio
estimated in K for the disk component was around
0.7 in solar units for all galaxies using the disk scale length
derived from the interarm regions.
Pitch angles of the main two-armed spiral estimated in B,V,I and
K show a systematic trend of arms being tighter
in bluer colors which suggests the presence of density waves in the
galaxies. Comparing the extent of the spiral pattern in the galaxies
with their angular velocity curves, the best agreement was obtained
when the symmetric, two-armed spiral starts just outside ILR and
terminates around the 4:1 resonance. The spirals continue beyond this
region but are weaker and more fragmented. The pattern speed estimated
for the two Sb galaxies was significantly higher than that for the Sc
galaxies.
Several galaxies have dust spirals inside the main stellar spiral. In most cases, the main spiral pattern is more open in the inner region than further out where it is well approximated with a logarithmic spiral. This may support a scenario where a central oval distortion drives the spiral, which then would correspond to the long waved solution of the dispersion relation.
Key words: galaxies: kinematics and
dynamics 
galaxies:
photometry
galaxies:
spiral
galaxies:
structure
infrared: galaxies
* Based on observations collected at the European Southern Observatory, La Silla, Chile
Send offprint requests to: P.J. Grosbol,(pgrosbol@eso.org)
Contents
- 1. Introduction
- 2. Data
- 3. Axisymmetric parameters
- 4. Morphology and spiral structure
- 5. Discussion
- 6. Conclusion
- Acknowledgements
- References
© European Southern Observatory (ESO) 1998
Online publication: July 27, 1998
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